1. Field of the Invention
The present invention generally relates to the field of electrical circuits. More particularly, the invention relates to amplifier circuits.
2. Background Art
Communication devices, such as cellular telephones, generally utilize power amplifiers that operate in a linear mode, where output power is controlled by input power, and power amplifiers that operate in a saturated mode, where input power is constant and output power is controlled by a control voltage, to support different modulation standards. For example, power amplifiers that can operate in a linear mode can be used in cellular telephones that use Enhanced Data Rates for GSM Evolution (EDGE) modulation, while power amplifiers that can operate in saturated mode can be use in cellular telephones that use Gaussian minimum shift keying (GMSK) modulation. However, it is desirable for power amplifiers in communication devices, such as cellular telephones, to be able to operate in either a linear or a saturated mode to accommodate different modulation standards, such as EDGE and GMSK.
Several conventional approaches have been made to provide a multimode power amplifier that can support both linear and saturated operating modes. One conventional approach utilizes a heterojunction bipolar transistor (HBT) amplifier incorporating a switched bias technique, where a low impedance voltage bias is provided for linear mode operation and a high impedance voltage bias is provided for saturated mode operation. However, the switched bias technique utilized in this approach can require a complex circuit to switch the bias between operating modes, which can undesirably result in an increased component count, compromised noise performance, and slew rate limits on mode transitions.
In another conventional approach to provide a multimode power amplifier, a low impedance voltage bias is maintained at the power amplifier for linear mode operation, while the power amplifier supply voltage is regulated to control the output power in saturated mode operation. While this approach avoids switching the bias between linear and saturated operating modes, it requires a high current low drop out regulator (LDO) to support the current requirements of the output stage amplifier and introduces an undesirable voltage drop in the supply voltage of the amplifier, thereby reducing overall amplifier efficiency and the linear operating mode performance.